1. Technical Field
Disclosed herein are methods and systems to receive and process optical signals, including free-space optical (FSO) signals, including methods and systems to control a gain applied to an optical signal in response to time-varying intensity fluctuations, to translate the optical signal to a substantially constant-intensity optical signal.
2. Related Art
Free Space Optics (FSO) systems are used for line-of-sight communications, and may be used over distances of several kilometers (km).
There is a move towards designing free space optical communications systems to couple received light into a single-mode fiber. This may permit relatively large optical bandwidths to be accessed using elements developed and utilized in terrestrial and sub-sea optical fiber networks, such as high density optical multiplexers and demultiplexers, optical attenuators, optical filters, and optical amplifiers.
Free space optical (FSO) links are inherently different than fiber optic links, in that a FSO communication link may suffer from attenuation effects, line-of-sight limitations, and/or optical turbulence along a beam path. In addition to inducing beam spread above a diffraction limit, turbulence may introduce disruptive intensity fluctuations at a receive terminal, where large power swings may occur in millisecond scales. Power collected at a single mode FSO terminal output may vary dynamically with, for example, greater than 40 dB swings in the received signal due to scintillation.
Attenuation changes may lead to excessive errors with intensity-modulated direct detection data communications, both when amplitude variations occur over a time scale comparable to the bandwidth of the decision circuits designed to determine the presence of a mark or a space, when variations couple through the decision electronics affecting the decision threshold, or when the undesirable amplitude modulation exceeds an amplitude range of decision circuits.
Attenuation changes may be exacerbated by conventional fixed gain optical pre-amplifiers, such as erbium-doped fiber amplifiers (EDFAs), because they may output power levels well above a damage threshold of a detector in response to rapid power transients in a “Q-switch” effect.
High power levels may exist, for example, where a distance between FSO communication systems are relatively close to one another, such as within approximately 10 kilometers (km), and may arise over longer distances during benign turbulence, such as up to 100 km distances.
There is also a possibility of damage to sensitive optical detectors in cases where the upper limit of the power variations exceeds the damage threshold of the optical detector. For example, commercially available high-sensitivity receivers, such as avalanche photodiodes, have relatively limited dynamic ranges and suffer from saturation and damage from large power variations.
Conventional FSO systems utilize front-end, time-dependent loss mechanisms, such as an attenuator, prior to a receive-side optical amplifier to stabilize energy received from a fiber-coupled free-space optical terminal. Attenuation devices, by design, decrease the optical signal level to the receiver, even at minimum attenuation, and may thus reduce the optical signal-to-noise ratio (OSNR), of the received signal from which data is extracted, and may reduce the power of a received signal at a receiver input below an optimal level.
Optical amplifiers include, among others, doped fiber amplifiers (DFAs) and Raman amplifiers.
DFAs use a doped optical fiber as a gain medium to amplify an input optical signal. The input optical signal and a pump laser are multiplexed into the doped fiber, where the input signal is amplified through interaction with the dopant ions. The pump laser excites dopant ions into a higher energy from where they decay via stimulated emission of a photon at a wavelength of the input signal wavelength, and return to their lower energy level.
In an erbium doped fiber amplifier (EDFA), a fiber core is doped with trivalent Erbium ions, and may be pumped with a laser at a wavelength of 980 nm or 1,480 nm, and may exhibit gain in the 1,550 nm region.
In addition to decaying via stimulated emission, electrons in the upper energy level may also decay by spontaneous emission, in which photons are emitted spontaneously, or randomly, in all directions. A portion of the spontaneously emitted photons may be amplified by other dopant ions via stimulated emission, and are thus referred to as amplified spontaneous emission (ASE). Forward-propagating ASE may co-propagate with the amplified input signal and may thus degrade amplifier performance. Backward or counter-propagating ASE may reduce a gain of the amplifier.
Raman amplifiers are based on stimulated Raman scattering (SRS) phenomenon, in which a photon of a lower frequency input optical signal induces inelastic scattering of a photon of a higher-frequency pump laser, within a non-linear gain medium lattice, such as an optical fiber. The inelastic scattering produces a photon coherent with the input optical signal, and resonantly passes surplus energy to vibrational states of the gain medium.
Raman amplifiers include distributed and lumped Raman amplifiers. In a distributed Raman amplifier, a transmission fiber is used as the gain medium. The transmission fiber may be a highly nonlinear fiber with a relatively small core to increase interactions between the input optical signal and the pump laswer and thereby reduce the length of fiber needed. In a lumped Raman amplifier, a dedicated, shorter length of fiber is used as the gain medium.
A Raman amplifier pump laser light may be coupled into the transmission fiber in the same direction as the signal (forward-pumped), in the opposite direction (reverse-pumped), or both.
A Raman amplifier pump laser may use more power than that of an EDFA for a given gain, but may provide more distributed amplification within a transmission fiber, which may increase the distance the amplified light can travel, and may provide amplification over a wider range of regions.